1. Field of the Invention
The present invention relates to an image forming apparatus which uses induction heating to heat a paper sheet onto which a toner image has been transferred and fix the toner image thereto, such as a multifunction peripheral, a copier, a printer, or a FAX machine.
2. Description of Related Art
Up to now, in some image forming apparatuses such as a multifunction peripheral, a formed toner image is transferred onto a paper sheet and fixed thereto by heat. A fixing device for fixing the toner image includes a rotary member such as a roller in order to perform heating while transporting the paper sheet. Some fixing devices fix the toner image by generating Joule heat by an eddy current due to a magnetic flux occurring from a coil or the like (i.e., by performing induction heating) and heating the rotary member to thereby heat the paper sheet. Here, it is necessary to keep heating the rotary member during printing because heat is taken away by a passing paper sheet from a portion (sheet passing region) of the rotary member heated by the induction heating which is in contact with the paper sheet. However, the temperature of a non-sheet passing region, which is not brought into contact with the paper sheet, keeps rising. For prevention of an excessive temperature rise in the non-sheet passing region or other such purpose, there is known a fixing device that adjusts a heating width while preventing a magnetic flux from reaching the non-sheet passing region of the rotary member.
For example, there is known a fixing device including: magnetic flux generating means for generating a magnetic flux; a heating element subjected to induction heating by the magnetic flux generated by the magnetic flux generating means; and an opposing core located so as to be opposed to the magnetic flux generating means across the heating element. The opposing core has a shape in which a clearance between an opposing portion that is opposed to a magnetic path of the magnetic flux and the heating element is smaller than a clearance between a non-opposing portion that is not opposed to the magnetic path and the heating element. The opposing portion of the opposing core is formed of a convex portion that protrudes from the non-opposing portion, and a length in a longitudinal direction of the convex portion corresponds to a width of a size of a recording medium that can be passed. A magnetic shielding member for shielding against the magnetic flux flowing in the non-sheet passing region is disposed to the opposing core so as to be opposed to the non-sheet passing region of the heating element.
There exists an image forming apparatus that changes the heating width according to the size of the paper sheet. However, in a case where the heating width remains changed until a print job is completed, the temperature drops in the non-sheet passing region of the rotary member. In this case, at the end of the print job, the temperature of the non-sheet passing region may become lower than a fixation control temperature that is necessary for fusing toner and is to be maintained in performing fixation. Therefore, when a new print job is executed by using a paper sheet having a larger size (for example, A3 size) than a paper sheet used for the preceding print job (for example, postcard printing), it is necessary to heat the entire sheet passing region of the paper sheet used for the new print job and raise a temperature thereof. This leads to a problem that a user needs to wait because of being unable to execute a print job immediately after completion of the preceding print job.
Further, there is a method of controlling the heating width according to a paper sheet size when the temperature of the non-sheet passing region rises during the printing. According to this method, it is true that the temperature of the non-sheet passing region may be prevented from becoming lower than the fixation control temperature at completion of a print job and that a new print job may be executed immediately after the completion of the preceding print job. However, under such control, the non-sheet passing region that does not need to be heated is actively heated, which poses a problem of wasteful power consumption.
Those fixing devices can perform heating control according to a region of the paper sheet. However, it is impossible to solve the problem that the temperature of the non-sheet passing region becomes lower at the completion of a print job or the problem that a new print job cannot be executed immediately after the completion of the preceding print job and that the user needs to wait. Further, it is also impossible to solve the problem of wasteful power consumption due to the heating of the non-sheet passing region that does not need to be heated.